HOW IT WORKS
Six beams, one trap
In about 1985 researchers crossed six laser beams tuned slightly below the colour sodium atoms absorb. Atoms moving any direction met photons that pushed them back.
See how coldLASER COOLING
Light that makes
things colder.
Tuned just right, laser light can slow atoms almost to a stop.
µK millionths of a degree
≈ 240 µK First optical molasses temperature
- Photons push
- Doppler picks
- Atoms slow
- Physics that won a Nobel Prize
At room temperature, the atoms and molecules in air move at about 4,000 km/h. Laser cooling brings atoms to within millionths of a degree of absolute zero, and the UK's most accurate clocks rely on it.
THE SHORT ANSWER
Push atoms
the right way.
A photon carries momentum, like a curling stone.
Absorbing one head-on slows an atom a little; millions of times over, a lot.
OPTICAL MOLASSES
A glowing pea
Where the beams met, the Nobel committee says, a cloud the size of a pea formed, of about a million chilled atoms, moving as if through thick liquid.
The numbers
IN YOUR LIFE
Keeping time
NPL's caesium fountain clocks use laser-cooled atoms to realise the SI second and help set the international time scale, UTC.
Where it reaches youA QUICK TEMPERATURE CHECK
How cold,
how fast?
Pick a stage to see the temperature and how fast the atoms move. The bar shows how close it is to absolute zero.
Figures from the Royal Swedish Academy of Sciences' 1997 Nobel Prize in Physics press release.
FOUR PLACES IT TOUCHES YOUR DAY
Cold atoms,
everyday time.
You never see laser-cooled atoms, but NPL says atomic timekeeping underpins technology most of us use daily.
The UK's official time
NPL operates the national time scale, UTC(NPL), and the UK's primary frequency standards.
Phones and the internet
Atomic timekeeping underpins mobile phones and the internet, and helps timestamp financial transactions.
Satellite navigation
Satellite navigation systems work out location from signals sent by atomic clocks on board the satellites.
A new second
NPL is developing optical clocks based on laser-cooled ions and atoms that already beat caesium standards, work expected to lead to a redefinition of the second.
TWO NUMBERS WORTH KNOWING
From jet speed
to a crawl.
One figure for the cold,
one for the clocks it makes possible.
≈ 240 µK
First optical molasses temperature
About 240 millionths of a degree above absolute zero, with sodium atoms slowed to about 30 cm/s. Gravity made them fall out of the beams in about a second.
THE COLD · NOBELPRIZE.ORG
< 5 × 10⁻¹⁶
Accuracy of NPL's caesium fountain standard
Letting laser-cooled atoms rise and fall like a fountain gives a long time to measure them, which is why these clocks are so precise.
THE CLOCKS · NPL
NPL says its first operational caesium-beam atomic clock, built in 1955, would gain or lose only one second in three hundred years, and modern atomic clocks can be as much as a million times more accurate.
THE TRICK
The Doppler
effect does it.
An atom only absorbs light of just the right colour. Tune the laser slightly too red and only atoms moving towards the beam see it as the right colour, just as a train's whistle sounds higher as it approaches. Those atoms absorb a photon and slow down.
The atom re-emits a photon about a hundred-millionth of a second later and can then absorb another, so the braking builds up quickly. The 1997 Nobel Prize in Physics recognised these methods to cool and trap atoms with laser light.
Questions
Questions people ask
How can a laser make something colder?
Photons carry momentum. If an atom moving towards a laser absorbs a photon of the right energy, it slows a little. Repeated many times, this can cool a gas to millionths of a degree above absolute zero.
What is optical molasses?
The region where crossing laser beams slow atoms so much that they move as if through thick liquid. The first one held about a million sodium atoms in a cloud the size of a pea.
How cold does laser cooling get?
The first optical molasses reached about 240 µK. Later work reached about 40 µK for sodium and about 2 µK for caesium, according to the Nobel Prize press release.
What is laser cooling used for?
NPL's caesium fountain clocks use laser-cooled atoms to realise the SI second and contribute to UTC. The Nobel committee also pointed to atom interferometers for very precise gravity measurements.
Will the definition of the second change?
NPL says optical atomic clocks based on laser-cooled ions and atoms already surpass caesium standards, and this work is expected to lead to a redefinition of the second.
Where the facts come from
Updated